Cellular (or mobile) telephone systems are assuming increasing importance in the art of telephony and may eventually displace a significant portion of fixed wireline service as it becomes technically more efficient.
Cellular telephony is based on radio frequency (RF) rather than wireline technology and therefore is subject to many problems which do not arise in wireline service.
Although mobile cellular systems have heretofore been based primarily on analog technology, this technology has severe limitations involving, inter alia, complexity, spectrum efficiency, privacy and cost. This has led to developments whereby digital technology, which is already in place for fixed telephone service, is now beginning to be substituted for the analog technology.
It is apparent that mobile systems will eventually include both the portable, or hand-carried, telephone type and the vehicle-mounted type. The present invention, although foreseeably adaptable to the portable type, is primarily concerned with the vehicle-mounted type of system.
One of the problems in RF based communication systems, especially when used in a mobile environment, is their susceptibility to fading and shadowing. This is a common phenomena which is often encountered in a vehicular-mounted radio where reception suddenly fades at one spot but is restored by a small movement of the vehicle.
A well-known technique to combat such fading and shadowing is the use of diversity. Two types of diversity heretofore used were time diversity which comprises the sending and receiving of the same message more than once and frequency diversity which comprises the sending and receiving of a message on more than one carrier frequency. However, both of these methods are subject to the drawback that they require additional bandwidth.
A third type of diversity, which does not require additional bandwidth, is spatial diversity. This comprises the use of two or more antennas that are separated from each other by an appropriate distance on the vehicle. Since the fading characteristics of these antennas are statistically independent of each other, when one is subject to fading the other can generally carry the full signal and thereby obviate the fading effects. However, these separate antennas provide separate signals which could result in duplication and interference with each other when not properly controlled.